The present invention relates generally to a measuring apparatus and an exposure apparatus having the same, which measures the optical performance of an optical element, such as a wavefront aberration and a Zernike coefficient. More specifically, the present invention relates to a measuring apparatus and an immersion exposure apparatus having the same, which measures the optical performance of a projection optical system in an immersion exposure apparatus.
An interferometer as an optical-performance measuring apparatus measures the optical performance of the projection optical system, without filling the fluid or liquid in the space between the projection optical system and the wafer, in the conventional immersion exposure apparatus. Rather, the space is filled with air and a large spherical aberration occurs due to a refractive index difference between the air and the fluid. In this case, the interference fringes are too dense to measure or to precisely measure. Therefore, the wavefront aberration is measured after a spherical aberration, a non-axial coma, and the like are minimized by adjusting the height of the object plane of the projection optical system or reticle surface, and the intervals among the optical elements, such as a lens and a mirror, in the projection optical system. Prior art include Japanese Patent Publications, Application Nos. 2001-074605 and 2002-071513, and 2002-250678.
Referring now to FIG. 7, a description will be given of details of a conventional optical-performance measuring apparatus. A target optical system 115 to be measured guides the light from a light source 101 having a good coherency and an oscillation wavelength close to a usable wavelength of the target optical system 15, such a laser light source, to an interferometer unit 102. The light from the light source 101 is split into measuring light and reference light in the middle of the optical path. As to the optical path of the measuring light, a condenser lens 103 condenses the light onto a spatial filter 104 in the interferometer unit 102. A diameter of a spatial filter 104 is set to about half an airy disc diameter determined by a numerical aperture (“NA”) of a collimeter lens 106. Thereby, the exited light from the spatial filter 104 becomes an ideal spherical wave, passes a half-mirror 105, is converted by the collimeter lens 106 into collimated light, and exits from the interferometer unit 102.
Then, the measuring light is guided to a top of the object plane of the target optical system 105 via a deflective optical system 110, and incident upon TS-XYZ stages 122-124. A mirror 111 fixed on the stage base 121 reflects the light in the Y direction, a Y moving mirror 112 on the TS-Y stage 122 reflects the light in the X direction, and a X-moving mirror 113 on a TS-X stage 123 reflects the light in the Z direction. A TS lens 114 on the TS-Z stage 121 condenses the light upon the object plane of the target optical system 115, and the light re-images on the image plane (wafer surface) via the target optical system 115.
The object plane shifts along an optical-axis direction to the reticle surface in the exposure apparatus. Since the fluid is not filled in the space, the spherical aberration occurs on the normal object plane position and a measurement of the precise wavefront aberration becomes difficult. The position of the object plane is set so that the spherical aberration is minimized.
Thereafter, an RS mirror 132 on RS-XYZ stages 125-127 reflects the light, and the light goes back to the interferometer unit 102 via the target optical system 115, the TS lens 114, the mirrors 113-111 and the deflective optical system 110.
The measuring light is incident upon the interferometer unit 102 and the collimeter lens 106, reflected by the half-mirror 105, and condensed upon the spatial filter 107. The spatial filter 107 shields the stray light and a steep slope wavefront. The light passes the spatial filter 107, and is incident upon the CCD camera 109 via the imaging lens 108.
On the other hand, the reference light is incident upon the TS lens 114 from the X moving mirror 113, and the part of the light is reflected on the TS lens 114. The surface reflection light from a Fizeau surface as a final surface of the TS lens 114 goes back along the same optical path, and is incident as the reference light upon the CCD camera 109. A superposition of the reference light and measuring light forms interference fringes.
The TS-XYZ stages 122-124 and RS-XYZ stages 125-127 move to an arbitrary image-point position of the target optical system 115 based on a command from the host computer 131 via a controller 130, a TS-XYZ stage driver 128, and an RS-XYZ stage driver 129.
This configuration enables the wavefront aberration to be continuously measured at arbitrary image points in the exposure area.
However, the above conventional optical-performance measuring apparatus has difficulties in canceling all the axial and non-axial aberrations, such as low and high orders spherical aberrations and non-axial coma, resulting in the increased aberrational residues as the target optical system has a high NA. As a result, the fringe interval in the interference fringes are denser than the spatial resolution of the interferometer and the wavefront aberration cannot be measured. The measuring accuracy generally lowers with a magnitude of the target wavefront, and the wavefront can be measured but not precisely.